The present invention relates to a light-sensitive composition which comprises an alkali-soluble resin, a compound which is decomposed by an acid and is made alkali-soluble and a specific compound which generates an acid upon irradiation with actinic rays or radiant rays, and which is sensitive to radiation such as ultraviolet rays, far ultraviolet rays, electron rays, molecular beam, .gamma.-rays and synchrotron radiation. In particular, the present invention relates to a photoresist, especially a photoresist composition suitable for fine processing which is high in image resolution and sensitivity and excellent in storage stability.
The photoresist of the present invention is applied to a substrate such as a semiconductor wafer, glass, ceramic and metal by a spin coating or a roller coating in thickness of 0.5 to 3 .mu.m, which is then heated and dried, and imagewise exposed to light through an exposure mask having an image such as a circuit pattern to form a positive image. Further, the positive image is used as a mask to etch the substrate to make the pattern on the substrate. Typical applications include a process for the production of a semiconductor element such as integrated circuits (IC), a process for the production of a circuit board such as liquid crystal and thermal head and other photofabrication processes.
As so-called positive working photoresists, there have been used compositions comprising an alkali-soluble resin and a naphthoquinonediazide compound as a light-sensitive material. For example, novolak phenol resin/naphthoquinone diazide substituted compounds are disclosed in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470 and as the most typical composition, an example of novolak resin comprising cresolformaldehyde/trihydroxy benzophenone-1,2-naohthoquinonediazide sulfonic acid ester is disclosed in L. F. Thompson "Introduction to Microlithography" (ACS Publishing, No. 2, 19, pp. 112-121).
Novolak resins used as a binder are suitable for this applications because they are soluble in alkaline water without swelling and highly resistant to plasma etching when the resulting resist image is used as a mask for the etching. Naphthoquinonediazide compounds are suitable for a light-sensitive material of the positive working photoresist because they change their property upon light exposure, i.e., they serve as a dissolution inhibitor to reduce alkali-solubility of the novolak resins but upon exposure to light, they decompose to form an alkali-soluble substance and serve as an accelerator to enhance alkali-solubility of the novolak resins.
There have been developed and practically used a number of positive working photoresists comprising novolak resins and naphthoquinonediazide compounds and sufficient results have been obtained for the processing of patterns having lines of 1.5 to 2.0.mu.m in width.
However, the packaging density of integrated circuits becomes higher and higher. It is required the processing of ultrafine patterns having lines of not more than 1 .mu.m in width in the production of substrates for semiconductor such as ultra LSI. In these applications, such photoresists are required that are high in image resolution, high in pattern reproduction to faithfully reproduce a pattern of an exposure mask, high in sensitivity from the view point of high productivity and excellent in storage stability.
There have been attempts to increase the content of quinonediazide light-sensitive radicals so as to obtain high image resolution. For example, J.P.KOKAI No. Sho 60-158440 discloses the use of a light-sensitive compound containing a higher amount of triester and J.P.KOKAI No. Sho 61-118744 discloses the attempt to increase the number of quinondiazide light-sensitive radicals contained in one molecule of a light-sensitive compound.
However, these light-sensitive compounds have disadvantages that they are insoluble in a solvent conventionally used, or soluble but form deposits during the storage of the photoresist compositions, which cause troubles in a photofabrication process or a device production process and reduce the yield (i.e., the number of IC chips obtained from one piece of silicon wafer). In order to prevent the deposition of the light-sensitive composition, J.P.KOKAI No. Sho 61-260239 discloses the use of an organic solvent having a solubility parameter of 11 to 12. However, since such a solvent as dimethyl sulfoxide is used, the resist composition is low in storage stability, i.e., the sensitivity and image resolution are significantly reduced over time.
It is also considered to use a light source emitting light of shorter wavelength so as to attain high image resolution. For instance, there have been studied to use conventional g-ray (436 nm), i-ray (365 nm), Deep-UV rays (200-300 nm) and KrF excimer laser (248 nm). In order to form a resist pattern having a good shape using the above-mentioned light sources, it is required that a resist composition is low in the absorption of light in these wavelength regions and high in bleaching property ("bleaching" means the reduction in the intrinsic absorption of a substance, in particular herein the reduction in the absorption in Deep-UV regions when exposed to light of Deep-UV regions, e.g., 248 nm) at the wavelength of light exposed. However, naphthoquinonediazide light-sensitive materials conventionally used are high in the absorption of light in Deep-UV regions and low in bleaching property at the wavelength and therefore they are insufficient to meet the requirements to make the wavelength of a light source shorter.
These quinonediazide compounds are decomposed by irradiation with actinic rays to form 5-membered carboxylic acids and they are thus made alkali-soluble. In these applications of the light-sensitive material, such properties of the compounds are utilised. However, their light-sensitivity is insufficient. This stem from the fact that their quantum yield essentially never exceeds 1.
Many attempts have been made to improve the light-sensitivity of light-sensitive compositions containing quinonediazide compounds. However, it is very difficult to improve the light-sensitivity while maintaining the development latitude during development.
On the other hand, there have been proposed some positive working light-sensitive compositions free of quinonediazide compounds. One example thereof comprises a polymeric compound having o-nitrocarbinol ester groups as disclosed in J.P. KOKOKU No. Sho 56-2696. However, such a composition does not provide high sensitivity for the same reasons as those discussed above in connection with quinonediazide compounds.
Separately, there have been proposed methods to improve light-sensitivity using a light-sensitive system which is catalytically activated, wherein a known principle is used that a photolytically generated acid causes a second reaction which makes resist in exposed areas soluble. Examples of the methods include combinations of photolytically acid producing compound and acetal or O- or N-acetal compound (J.P.KOKAI No. Sho 48-89003), orthoester or amideacetal compound (J.P.KOKAI No. Sho 51-120714), polymer having in the main chain acetal or ketal groups (J.P.KOKAI No. Sho 53-133429), enolether compound (J.P.KOKAI No. Sho 55-12995), N-acylimino carbonic acid compound (J.P.KOKAI No. Sho 55-126236), polymer having in the main chain orthoester groups (J.P.KOKAI No. Sho 56-17345), silyl ester compound (J.P.KOKAI No. Sho 60-10247), and silyl ether compound (J.P.KOKAI Nos.Sho 60-37549 and 60-121446). Since quantum yield principally exceeds 1 in these combinations, high light-sensitivity is realized.
There have been also proposed systems which are stable over time at room temperature but are decomposed by heat in the presence of an acid to become alkali-soluble. Examples of such systems include a combination of a compound which produces an acid upon exposure to light and secondory or tertiary carbon (e.g. t-butyl or 2-cyclohexenyl) ester or carbonic acid ester compound disclosed in J.P.KOKAI Nos. Sho 59-45439, 60-3625, 62-229242, and 63-36240, Polym. Eng. Sci., vol. 23, page 1012, (1983), ACS.Sym.,vol.242, page 11 (1984), Semiconductor World (1987), November, page 91, Macromolecules, vol. 21, page 1475 (1988) and SPIE, vol. 920, page 42 (1988). These systems are also high in sensitivity and lower in absorption in Deep-UV regions than naphthoquinonediazide/novolak resin systems so that they can be a useful system wherein a wavelength of a light-source can be made shorter. However, since the acid generating agent used in combination does not cause photo-bleaching, the systems are not sufficient to obtain a resist pattern having a good shape.
A system comprising a combination of a silyl ether compound and a disulfone compound as an acid generating agent is disclosed in J.P.KOKAI No. Sho 61-166544 and German patent No. 3,804,533. However, the former system has disadvantage that the absorption in Deep-UV regions is large and the latter has disadvantage that alkali developability of exposed areas and dissolution inhibition of unexposed areas in the developer are not sufficient.